(1) Field of the Invention
The present invention relates to a plain bearing for supporting a crank shaft of an internal combustion engine and to a bearing device.
(2) Description of Related Art
From the past, a plain bearing constituted by a pair of semicylindrical members has been employed as a main bearing and as a connecting rod bearing. A so-called crush relief is formed in the plain bearing at an inner circumferential surface adjacent to contact surfaces of the semicylindrical members.
A crush relief is a wall-thickness thinner region that is formed in a region adjacent to a circumferential end surface of a semicylindrical member such that a thickness of a wall is thinner toward the circumferential end surface. A crush relief is formed with an intention of absorbing misalignment or deformation of joining surfaces of the semicylindrical members when the pair of semicylindrical members are assembled (see, for example, Japanese Utility Model Laid-Open No. 2-102014).
In recent years, the weight of an internal combustion engine has been reduced in order to improve fuel efficiency, and thus stiffness of a housing member, such as a connecting rod and an engine block, is being reduced. Thus, a bearing holding hole deforms elastically due to a dynamic load from a crank shaft when operating an internal combustion engine. Therefore, a plain bearing follows the deformation of the bearing holding hole, which results in a phenomenon in which a clearance in the horizontal direction of the plain bearing (a space between the inner circumferential surface of the plain bearing and a surface of a mating shaft) increases and decreases repeatedly (a close-in phenomenon). In a case of a connecting rod or an engine block of which stiffness has been reduced, an amount of reduction in the clearance of the plain bearing (in a region near the joining surfaces of the pair of semicylindrical bearings) in the horizontal direction is large when the dynamic load from the crank shaft acts thereon. Then, the inner circumferential surface of the bearing comes into direct contact with the surface of the crank shaft at the circumferential end of the semicylindrical bearing which is at the forward side in a rotation direction of the crank shaft, whereby the bearing is damaged.
As a countermeasure to the above, in the past, as disclosed in JP-A-10-325410, a bearing space between a surface of a crank pin and the inner circumferential surface of the bearing at a circumferential end region of the bearing is increased to prevent a contact between the surface of the crank pin and the inner circumferential surface of the plain bearing caused by the close-in phenomenon, by forming an inner circumferential surface of a plain bearing in an arc shape in which an inner diameter is greater in the horizontal direction than in the vertical direction.
However, in recent years, the size of an oil pump has been reduced in an internal combustion engine, and thus an amount of lubricating oil that is supplied to the inner circumferential surface of a bearing is being decreased. In a case of a bearing in which the bearing space is increased as described in JP-A-10-325410, an amount of lubricating oil that leaks through the space is increased, which leads to a short supply of the lubricating oil to the inner circumferential surface of the bearing. Currently, it is hard to employ such a plain bearing as described in JP-A-10-325410.
It is an object of the present invention to provide a plain bearing that is capable of reducing an amount of lubricating oil that leaks out and preventing damage to the inner circumferential surface of the bearing even when a close-in phenomenon occurs while driving an internal combustion engine.